mimxrt/systick: Remove unused systick helper functions.

These legacy functions were copied verbatim from the stm32 port and never
used.  And the use of WFI in `systick_wait_at_least()` is probably wrong.

And correct the comment in SysTick_Handler, which was also copied from
stm32.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

ports/mimxrt/systick.c

@@ -36,9 +36,7 @@ volatile uint32_t systick_ms = 0;
 systick_dispatch_t systick_dispatch_table[SYSTICK_DISPATCH_NUM_SLOTS];
 
 void SysTick_Handler(void) {
-    // Instead of calling HAL_IncTick we do the increment here of the counter.
-    // This is purely for efficiency, since SysTick is called 1000 times per
-    // second at the highest interrupt priority.
+    // Increment the systick millisecond counter.
     uint32_t uw_tick = systick_ms + 1;
     systick_ms = uw_tick;
 
@@ -52,16 +50,3 @@ void SysTick_Handler(void) {
         pendsv_schedule_dispatch(PENDSV_DISPATCH_SOFT_TIMER, soft_timer_handler);
     }
 }
-
-bool systick_has_passed(uint32_t start_tick, uint32_t delay_ms) {
-    return systick_ms - start_tick >= delay_ms;
-}
-
-// waits until at least delay_ms milliseconds have passed from the sampling of
-// startTick. Handles overflow properly. Assumes stc was taken from
-// HAL_GetTick() some time before calling this function.
-void systick_wait_at_least(uint32_t start_tick, uint32_t delay_ms) {
-    while (!systick_has_passed(start_tick, delay_ms)) {
-        __WFI(); // enter sleep mode, waiting for interrupt
-    }
-}

ports/mimxrt/systick.h

@@ -54,7 +54,4 @@ static inline void systick_disable_dispatch(size_t slot) {
     systick_dispatch_table[slot] = NULL;
 }
 
-void systick_wait_at_least(uint32_t stc, uint32_t delay_ms);
-bool systick_has_passed(uint32_t stc, uint32_t delay_ms);
-
 #endif // MICROPY_INCLUDED_MIMXRT_SYSTICK_H